This invention relates to compounds useful for complexing halogens and more particularly to compounds useful as an electrolyte additive for electrochemical cells and batteries of the type which utilize a halogen as its electrochemically active agent.
Voltaic cells which include an aqueous solution of zine or cadmium halide as an electrolyte are known, but are frequently characterized by a relatively high self-discharge rate, low capacity, and high internal resistance. Since elemental halogen is soluble in the aqueous electrolyte, it is difficult to keep metallic zinc or cadmium and elemental halogen apart while simultaneously achieving a system in which a good percentage of the theoretical energy storage capacity can be realized.
Recently, various attempts have been made to prevent elemental halogen from migrating to the zinc or cadmium electrode. For example, U.S. Pat. No. 3,352,720 to G. R. Wilson et al. teaches the use of water insoluble polymeric amine halogen complexes in place of the elemental halogen. While the structures disclosed by Wilson et al. function as halogen cells, they nevertheless suffer from less than optimum capacities and self-discharge rates because of the low stability of the polyhalogens employed.
U.S. Pat. No. 3,816,177 to Myles A. Walsh teaches the use of soluble quaternary ammonium halides and the like which may be dissolved in the electrolyte together with a water soluble depolarizer. When elemental halogen is released into the electrolyte, it combines with the quaternary halide to form a quaternary polyhalide which complexes with the depolarizer to form an insoluble, halogen rich, oil-like complex. If an inert electrode made of a material which absorbs the insoluble complex is employed, an improved cell is provided, since the complex is relatively stable and since the halogen molecules, being concentrated about the current collector, are available for electrochemical reaction to an improved degree.
Still another improvement in halogen electrochemical cells is disclosed in U.S. Patent Application Ser. No. 652,780 to M. A. Walsh et al. entitled Halogen Electrode, filed on Jan. 27, 1976. The structure disclosed by this application is a novel electrode which comprises a current collecting matrix which has an extremely stable polymer containing quaternary ammonium, phosphonium, or sulfonium sites incorporated into its structure. Since the polymer is formed in the presence of a high surface area, porous, current collecting matrix, the electrodes are capable of storing halogens in a non-chemically active but highly electrochemically active state in such a manner that uniform, intimate electrical contact is maintained between the halogen-rich sites of the polymer and the current collector, and interaction between halide in the electrolyte and the quaternary sites is greatly facilitated.
The instant invention constitutes a further improvement in the halogen cell art and provides a large number of compounds, any one of which may be added to the electrolyte of halogen cells of the type described. The compounds of the invention eliminate the necessity of a depolarizer or a specially fabricated electrode, yet provide increased halogen complexing ability, enhancing the shelf-life and capacity of the cells and batteries in which they are used.